Solubility and hydrolysis of Zr oxides: a review and supplemental data

Abstract

Summary Thermodynamic data for Zr(IV) have been reviewed in the context of a recent update of the Nagra/PSI thermochemical database, which is primarily used in safety assessment calculations for radioactive waste repositories in Switzerland. In spite of the important role of Zr in nuclear reactor technology and as fission product in radioactive waste, the thermodynamic properties of this element in aqueous solutions are poorly constrained and published data are frequently contradictory. Specifically, hydrolysis constants, which are critical to any geochemical calculation of Zr speciation in aqueous solutions, may vary by several orders of magnitude. Published solubility and hydrolysis data for Zr oxides were reviewed and evaluated. Recent solubility measurements on crystalline ZrO2, obtained in our laboratory, were integrated in this analysis. These new data are the only ZrO2 solubility measurements obtained in a pH region relevant for environmental aqueous systems, where equilibrium concentrations decrease to nanomolar levels. All solubility data of (hydrous) Zr oxides can be reproduced satisfactorily by a single set of hydrolysis constants, provided that only data measured from undersaturation are considered. This set includes the species Zr4+, ZrOH3+, Zr(OH)4 0 and Zr(OH)5 −. However, present experimental data are still not sufficient to establish a fully reliable database.